Magnetic random access memory array with free layer locking mechanism and method of its use

ABSTRACT

A method of using an MTJ MRAM cell element having two magnetization states of greater and lesser stability. During switching, the free layer is first placed in the less stable state by a word line current, so that a small bit line current can switch its magnetization direction. After switching, the state reverts to its more stable form as a result of magnetostatic interaction with a SAL (soft adjacent layer), which is a layer of soft magnetic material formed on an adjacent current carrying line, which prevents it from being accidentally rewritten when it is not actually selected and also provides stability against thermal agitation.

This is a Divisional Application of U.S. patent application Ser. No.10/818,581, filed on Apr. 06, 2004, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the design and fabrication of a magnetictunnel junction (MTJ) MRAM array, particularly to a design for locking(creating a stable magnetization state) un-selected array devices andunlocking (creating a less stable magnetization state) selected arraydevices.

2. Description of the Related Art

The magnetic tunnel junction (MTJ) basically comprises two electrodes,which are layers of ferromagnetic material, separated by a tunnelbarrier layer, which is a thin layer of insulating material. The tunnelbarrier layer must be sufficiently thin so that there is a probabilityfor charge carriers (typically electrons) to cross the layer by means ofquantum mechanical tunneling. The tunneling probability is spindependent, however, depending on the availability of tunneling stateswith different electron spin orientations. Thus, the overall tunnelingcurrent will depend on the number of spin-up vs. spin-down electrons,which in turn depends on the orientation of the electron spin relativeto the magnetization direction of the ferromagnetic layers. Thus, ifthese magnetization directions are varied for a given applied voltage,the tunneling current will also vary as a function of the relativedirections. As a result of the behavior of an MTJ, sensing the change oftunneling current for a fixed potential can enable a determination ofthe relative magnetization directions of the two ferromagnetic layersthat comprise it. Equivalently, the resistance of the MTJ can bemeasured, since different relative magnetization directions will producedifferent resistances.

The use of an MTJ as an information storage device requires that themagnetization of at least one of its ferromagnetic layers can be variedrelative to the other and also that changes in the relative directionscan be sensed by means of variations in the tunneling current or,equivalently, the junction resistance. In its simplest form as a twostate memory storage device, the MTJ need only be capable of having itsmagnetizations put into parallel (low resistance) or antiparallel (highresistance) configurations (writing data) and that these twoconfigurations can be sensed by tunneling current variations orresistance variations (reading data). In practice, the freeferromagnetic layer can be modeled as having a magnetization which isfree to rotate but which energetically prefers to align in eitherdirection along its easy axis (the direction of magnetic crystallineanisotropy). The magnetization of the fixed layer may be thought of asbeing permanently aligned in its easy axis direction. When the freelayer is anti-aligned with the fixed layer, the junction will have itsmaximum resistance, when the free layer is aligned with the fixed layer,the minimum resistance is present. In typical MRAM circuitry, the MTJdevices are located at the intersection of current carrying lines calledword lines and bit lines. When both lines are activated, the device iswritten upon, ie, the magnetization direction of its free layer ischanged. When only one line is activated, the resistance of the devicecan be sensed, so the device is effectively read. Such an MTJ device isprovided by Gallagher et al. (U.S. Pat. No. 5,650,958), who teach theformation of an MTJ device with a pinned ferromagnetic layer whosemagnetization is in the plane of the layer but not free to rotate,together with a free magnetic layer whose magnetization is free torotate relative to that of the pinned layer, wherein the two layers areseparated by an insulating tunnel barrier layer.

In order for the MTJ MRAM device to be competitive with other forms ofDRAM, it is necessary that the MTJ be made very small, typically ofsub-micron dimension. Parkin et al. (U.S. Pat. No. 6,166,948) teachesthe formation of an MTJ MRAM cell in which the free layer is formed oftwo antiparallel magnetized layers separated by a spacer layer chosen toprevent exchange coupling but to allow direct dipole coupling betweenthe layers. The free layer thereby has closed flux loops and the twolayers switch their magnetizations simultaneously during switchingoperations. Parkin notes that sub-micron dimensions are needed to becompetitive with DRAM memories in the range of 10-100 Mbit capacities.Parkin also notes that such small sizes are associated with significantproblems, particularly super-paramagnetism, which is the spontaneousthermal fluctuation of magnetization in samples of ferromagneticmaterial too small to have sufficient magnetic anisotropy (a measure ofthe ability of a sample to maintain a given magnetization direction). Toovercome the undesirable spontaneous thermal fluctuations in MRAM cellswith very small cross-sectional areas, it is necessary to make themagnetic layers thick. Unfortunately, the size of the switching fieldincreases with layer thickness, so the price paid for a thermally stablecell is the necessity of expending a great deal of current to change themagnetic orientation of the cell's free layer.

Some degree of anisotropy is necessary if an MTJ cell is to be capableof maintaining a magnetization direction and, thereby, to effectivelystore data even when write currents are zero. As cell sizes havecontinued to decrease, the technology has sought to provide a degree ofmagnetic anisotropy by forming cells in a wide variety of shapes (eg.rectangles, diamonds, ellipses, etc.), so that the lack of inherentcrystalline anisotropy is countered by a shape anisotropy. Yet this formof anisotropy brings with it its own problems. A particularlytroublesome shape-related problem in MTJ devices results fromnon-uniform and uncontrollable edge-fields produced by shape anisotropy(a property of non-circular samples). As the cell size decreases, theseedge fields become relatively more important than the magnetization ofthe body of the cell and have an adverse effect on the storage andreading of data. Although such shape anisotropies, when of sufficientmagnitude, reduce the disadvantageous effects of super-paramagnetism,they have the negative effect of requiring high currents to change themagnetization direction of the MTJ for the purpose of storing data.

Another way to address the problem that high currents are needed tochange the magnetization direction of a free layer when the shapeanisotropy is high, is to provide a mechanism for concentrating thefields produced by lower current values. This approach was taken byDurlam et al. (U.S. Pat. No. 6,211,090 B1) who teach the formation of aflux concentrator, which is a soft magnetic (NiFe) layer formed around acopper damascene current carrying line. The layer is formed around threesides of the copper line which forms the digit line at the underside ofthe MRAM cell.

In conventional MRAM design, both the word and bit lines that intersectat the location of a particular MRAM cell must be carrying currents inorder for that selected cell to be switched and for a 0 or a 1 to bewritten thereon. For the large number of other cells that lie along onlythe current carrying bit line, or along only the current carrying wordline, but not at their intersection, only the field of a single line isexperienced. Such cells are called half-selected. In an MRAM array, thehalf-selected cells must not be switched and the selected cell must beswitched.

Every cell in an array can be thought of as being potentially under theinfluence of the superposition of two magnetic fields. Ideally, if onlyone line is carrying current, the local field at the cell positionshould be insufficient to switch the cell and the cell can be said to bein the un-switched zone of the local magnetic field. A problem arises,however, because variations in cell formation, particularly when cellsare extremely small, can allow a cell to switch even when it is in theun-switched zone of the local magnetic field.

The purpose of the present invention is to create a design margin foreach cell, so that individual variations in cell structure would beinsufficient to place a non-selected cell within the switched zone ofits local magnetic field. Such a design margin can be achieved if themagnetization of a cell can be locked into a highly stable “C” stateafter each act of writing on the cell (holding it in a stable stateduring write operations on other, nearby, cells), and if the cell canthen be placed into a less stable “S” state when it is actually beingwritten upon. In the method of the present invention, the capability ofa free layer to be placed into C or S states is provided bymagnetostatically coupling it to a soft magnetic layer (a claddinglayer) formed around a bit line and by building in a small amount ofmagnetic anisotropy into the free layer. This additional magnetostaticinteraction, along with the conventional magnetic fields produced bycurrents in the word and bit line, produces two states of flux closurewithin the free layer, which are the C and S states that are desired.

Cladding layers surrounding current carrying write lines have beentaught by others. Bloomquist et al. (U.S. Pat. No. 6,661,688 B2) teachesa write line structure in which a cladding layer nearly completelysurrounds a write line below a memory storage device. The cladding layerhas an open space above the write line, so that it effectively forms twopoles immediately adjacent to the storage device. The structure is saidto provide a greater field at the storage device for a given current inthe write line.

Bhattacharayya et al. teaches an array of magnetic memory cells whichare written upon by segmented write lines. The line segments are eachcladded with high permeability, soft magnetic material to increase theirmagnetic fields for given currents.

Sharma et al. (U.S. Pat. No. 6,593,608 B1) uses a cladded bit line toserve as a seed layer for the formation of a soft magnetic referencelayer (ie a pinned layer) within a double reference layer magneticmemory cell.

Jones et al. (U.S. Pat. No. 6,555,858 B1) discloses a self aligned cladbit line structure formed within a trench.

Rizzo (U.S. Pat. No. 6,430,085 B1) teaches a method of forming acladding layer of magnetic material so that the layer has a shapeanisotropy parallel to the conducting line that it clads and also has aninduced anisotropy that is not parallel to the shape anisotropy. Thecombination of the two anisotropies enhances the permeability of thelayer, thereby increasing the magnetic field for a given current.

Although all of the foregoing cited prior art teaches the use of amagnetic cladding layer for the purpose of enhancing the magnetic fieldsused for switching an MRAM cell, none of the prior art teaches the useof such cladding to magnetostatically couple to a free layer so as tocreate states of greater and lesser stability.

SUMMARY OF THE INVENTION

A first object of this invention is to provide an MTJ MRAM cell and anarray of such cells, that have states of free layer magnetization whichhave increased stability against unintended switching by thermalagitation and by the ambient fields of nearby write lines.

A second object of this invention is to provide an MTJ MRAM cell and anarray of such cells, which are more stable against switching of freelayer magnetization when not selected (ie unintended switching) to bewritten on but which can be rendered less stable when they are actuallybeing written upon.

A third object of this invention is to provide a word and bit lineconfiguration and a current scheme within these lines that can producemore stable and less stable magnetization states and can switch themagnetization state of an MRAM cell free layer.

These objects will be achieved by a novel MRAM cell design in whichvariations in free layer magnetization stability as a function ofselected or non-selected writing is achieved by the use of a compositebit line in which the current is carried by layers of high conductivitymaterial, and the layers are clad by an adjacent layer formed of a thin,soft-magnetic material (a “soft adjacent layer” or SAL) which providesmagnetostatic coupling with the free layer.

Referring to schematic FIG. 1 a, there is shown a cross-sectional view(in the xz plane) of an MTJ cell (50) placed between crossing word andbit lines. The word line (10) runs in y-direction (out of the figureplane), the bit line (20) runs in the x direction and the layers of thecell are stacked in the z-direction. The bit line is formed as upper(22) and lower (26) conducting layers, with a layer of soft magneticmaterial (24) (the SAL) between them. The lower layer (26) can bepresent or absent. The word line (10) is shown formed of two portions, aconducting layer (43) and a soft magnetic cladding layer (44). Thecladding layer on the word line may be present to enhance the wordline's magnetic field, but its presence or absence is not a part of thepresent invention. As will be discussed more fully below, the cellincludes a seed layer (30), an antiferromagnetic pinning layer (32), apinned layer (34), an insulating tunneling barrier layer (36), a freelayer (38) a capping layer (40) and a read word line (42), whoseoperation is not an important part of the invention. The pinned layercan be a synthetic ferrimagnetic layer or it can be a single layer.

FIG. 1 b shows, schematically, an overhead view of two MTJ cells (50,51) formed, respectively, between two parallel upper word lines (10, 11)and a single common lower bit line (20). The shape of the cell, inhorizontal cross-section as shown, has a larger radius of curvature atits bottom portion (7) than at its upper portion (9), giving it asomewhat triangular shape. This asymmetry of shape produces a magneticanisotropy in the free layer of the cell, so that the magnetization atthe upper narrow portion (9) is more easily changed than that of thelower, broader, portion (ie., the self-demagnetization field at thebroader lower portion is higher than at the narrower upper portion).This shape anisotropy is important in helping to create the two statesof the free layer when the layer couples magnetostatically with themagnetization of the SAL. In the figure, cell (50) beneath word line(10) has its free layer magnetization in a locked C state, cell (51)beneath word line (11) has its free layer in an unlocked S state. Thethree arrows within the cells (50) and (51), denote the magnetizationdirections within the upper portion (19), the most substantial centralportion (18) and the lower portion (17) of the free layer. The arrowdrawn within the bit line (21) represents a magnetic field in the SAL,which was presumably induced by current in word line (10) in the −ydirection. The direction of this magnetic field is used to lock orunlock the magnetization state of the free layer (as will be explainedin greater detail below) so that a current in the bit line can reverseit. The particular upper and lower arrow directions (19) and (17) in theunlocked free layer (51) make the magnetization directional switch ofthe central magnetization (18) much easier.

The SAL will be magnetized in the −x direction when the current in theword line is in the y-direction, and will be magnetized in the +xdirection when the current in the word line is in the −y direction.Because the SAL extends along the x-axis direction of the bit line, itsmagnetization tends to line up along this direction. Thus, when there iscurrent in the bit line (the +x-direction), which by itself produces amagnetic field in the y-direction, the magnetic field in the SAL hasboth x- and y-components.

Referring to FIG. 2, there is shown a bit line (20) within which thereis an outlined region (60) that is just below an MTJ cell. The freelayer of the cell (38) is shown above the bit line and is magnetized(arrow) along its easy-axis direction (+y). Three-dimensionally, thefree layer would be vertically (the +z direction) directly above theregion of the bit line (60). In the absence of any other fields inducedwithin the SAL, the magnetization of the free layer (up-arrow) wouldcreate a mirror image magnetized region (down-arrow) within the SAL bymagnetostatic coupling. However, the region (60) of the SAL ismagnetized in the direction shown (a slight negative y-component) by thearrow within it due to the combination of the field produced by the wordline (+x) and the magnetostatic coupling of the SAL to the free layermagnetization.

Within the parameters of this design both the free and pinned layers canbe a single ferromagnetic layer or a synthetic ferrimagnetic layer. Theadditional SAL is formed on the bit line and is patterned with it. Thesoft magnetic material of the SAL can be Ni, Fe, Co and their alloys,while the conducting material can be high conductivity materials such asAl, Cu, Au, Ru, Ta, CuAu or Rh. The switching current flowssubstantially through the high conductivity material, so the SAL can bemade very thin.

FIG. 3 shows schematic graphs of word line current I_(w) and bit linecurrent, I_(b), as a function of time during and just after a rewritingprocess applied to a particular cell. In the description of thepreferred embodiment below, this figure will be explained in greaterdetail with further amplification by the free layer magnetization statesof FIG. 4 a-d. We will assume the word line lies along the y-axis and apositive value of its current, I_(w), is in the −y (negative-y)direction. The bit line and the SAL formed on it, lie along the x-axisdirection. We also assume that the cell is already in a stable C state,because it had previously been written on.

During a first portion of the rewrite process, after time t₁, there is apositive current in the word line but no current in the bit line. Thecurrent in the word line induces a magnetic field in the SAL that is inthe +x direction. This is true for every cell that is below the activeword line. The induced SAL magnetization couples with the easy axismagnetization of the free layer, which we will assume is also in the +ydirection, and tilts it vertically, changing its state from stable C, toless stable S. The cell is now unlocked and ready to be written on.

To write on the unlocked cell, the bit line is activated at time t₂ andthere is simultaneously current in both the word and bit lines at thelocation of the selected cell. Because it is in the S state, the smallorthogonal magnetic field produced by the bit line current is sufficientto switch the free layer magnetization of the selected cell. Note thatwhen in the C state, the magnetic stability of the free layer preventsit from being switched with the currents typically carried by word andbit lines.

After switching, the word line current is reduced and reversed indirection and the bit line current is turned off. The interval duringwhich the word line current is reversed, but the bit line current isoff, after t₄, changes the free layer magnetization to the C state. Atthe same time, all the cells beneath the word line that were unlockedbut not rewritten are now restored to their C states.

Because of the shape induced magnetic anisotropy of the free layer, thesmall reversed current in the word line only changes the magnetizationof the free layer at its upper portion, which causes the switch from Sto C state. Finally, both currents are off and the magnetization revertsto the easy axis direction. Note that the switching from C to S and fromS to C occurs when only the word line is activated. The switching of themagnetization direction occurs only when both word and bit lines areactivated. The process just described will be discussed in greaterdetail below within the description of the preferred embodiment and withreference to FIGS. 4 a-d.

The SAL plays several roles in the invention. First, because of its highpermeability, the SAL concentrates the magnetic field produced by thecurrent in the bit line and the proximity of the SAL to the free layermakes the enhanced field extremely effective in switching as a result ofmagnetostatic coupling between the SAL and the free layer. Second, theSAL creates a magnetostatic coupling anisotropy and maintains C and Sstates for the free layer magnetizations of all cells, even when the bitline carries no current.

As was already seen in schematic FIG. 2 and discussed above, themagnetized region (60) of the SAL beneath a particular asymmetric MRAMcell free layer (38) tends to have its magnetization line up tiltedrelative to the bit line (20) direction when the magnetic field in thebit line is in the direction of the solid arrows (x-direction). The cellfree layer has both a built-in shape anisotropy because of itsapproximately triangular cross-section, and also an induced interactionanisotropy due to its magnetostatic coupling to the SAL beneath thecell. This interaction anisotropy is controlled by M_(s)t (product ofmagnetic moment and thickness) of the free layer and the SAL and thespacing between them. This interaction anisotropy can be preciselycontrolled by the fabrication process. During the writing process theword line write current generates a magnetic field along the bit linewhich will line up the magnetization of the SAL under the free layer inthat same direction. The magnetization of the SAL will rotate themagnetization of the free layer towards that direction. Then a small bitline current will rotate the magnetizations of the SAL and free layer inopposite directions. Removing the write current and then the bit linecurrent in sequence will leave the magnetizations of the free layer andSAL layer coupled by their mutual dipole interaction with themagnetization of the free layer controlled by the direction of the bitline current. To help maintain the free layer magnetization in the ydirection, the shape anisotropy has been introduced into the cell duringfabrication by giving the cell its slight triangular shape, as shown.Alternatively, some anisotropy can be given to the cell free layerduring the anneal to set the pinned layer magnetization.

Using the dipole-dipole interaction as a model for the magnetostaticcoupling between the free layer and the SAL, it can be shown that theinteraction anisotropy, K_(in), is proportional to:K _(in) ∝M _(s)(SAL)×M _(s)(free)×t _(SAL) ×a ² ×r ⁻³where a is the diameter of the cell, t_(SAL) is the thickness of theSAL, r is the distance between the free layer and the cell, and M_(s) isthe magnetic moment. The extreme sensitivity (inverse third power) to rshows that the bit line needs to be thin and close to the free layer.Also, because it is the bit line current that is responsible for theswitching, the current must be substantially in the highly conductivebit line layer. If it is desired to reduce the interaction anisotropy,the deposition process induced anisotropy and/or the shape inducedanisotropy can be set along the bit line direction (x), since theseanisotropies subtract from the interaction anisotropy. Finally, analternative design to enhance the word line field efficiency would be toadd a magnetic cladding layer over the word line on the side away fromthe cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows, schematically, a vertical cross-section of an MTJ cellformed between a word line and a bit line with a SAL.

FIG. 1 b shows, schematically, an overhead view of two cells formedbeneath parallel word lines and a single common bit line formed beneaththem.

FIG. 2 shows, schematically, a SAL and an MTJ cell free layer above it,showing the magnetizations in the free layer and in the region of theSAL just beneath the cell.

FIG. 3 shows, schematically, graphs of current vs. time for word lineand bit line currents during a write process on a cell.

FIGS. 4 a-d show the magnetization of a selected free layer as afunction of word and bit line currents shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention provides an MTJ MRAMcell and its method of use, or an MRAM array of such cells, in which therequired switching current in the bit line can be reduced, the bit stateof the cell rendered thermally stable between writing and rewriting andthe cells can be locked or unlocked in accordance with whether or notthey have been selected to be written upon, all by the addition of athin adjacent soft magnetic layer formed on the bit line which couplesmagnetostatically to a proximal free layer of the MTJ cell. Thepreferred embodiment also teaches a sequence of word and bit linecurrents that rewrite the MTJ MRAM cells in accord with the objects ofthe invention.

Referring back to FIG. 1 a there is shown in a schematic verticalcross-sectional view (in the xz-plane) the general configuration of theMRAM cell of the present invention. The MTJ element (50), which can havea horizontal cross-sectional shape to provide a magnetic anisotropy (andin this embodiment is given a somewhat curved triangular shape), issituated at a junction between a horizontal word line (20), which isabove the cell and runs out of the plane of the figure (in they-direction), and a horizontal bit line (30), which is below the celland runs in a direction (the x-direction) perpendicular to the wordline. The cell layers are stacked vertically in the z-direction. Thecombination of the MTJ element and the word and bit lines that accessand switch the cell form an MTJ MRAM cell.

The bit line is a composite layer formed as upper (22) and lower (26)conducting layers, with a layer of soft magnetic material (24) (the SAL)between them. The conducting layers carry substantially all of the bitline current. As will be discussed more fully below, the cell includes aseed layer (30), an antiferromagnetic pinning layer (32), a pinnedlayer, which in this embodiment is a synthetic ferrimagnetically coupledlayer comprising a second (33) and first (35) ferromagnetic layercoupled by a non-magnetic conducting layer (34), an insulating tunnelingbarrier layer (36), a free layer (38), which can be a multilayer, acapping layer (40) and a read word line (42), whose operation is not animportant part of the invention. The pinned layer can be a syntheticferrimagnetic layer or it can be a single layer.

The bit line typically is formed in a trench in a dielectric layer overa silicon substrate, but those details are not shown and are notnecessary to explain the preferred embodiment. The conducting layers (22and 26) of the bit line are formed of a non-magnetic high conductivitymaterial, such as Cu, Au, Al, Ag, CuAg, Ta, Cr, NiCr, NiFeCr, Ru, Rh andtheir multi-layers and alloys. Since the upper layer (22) separates theSAL from the free layer of the cell, it must be as thin as possible,less than 1000 angstroms, for optimal coupling between the SAL and thefree layer. In addition, the width of the bit line should be greaterthan 50% of the lateral dimension of the cell. The lower conductinglayer (26) can have a thickness between 0 and 1000 angstroms, the zerovalue indicating that it can be omitted. If the lower layer is present,a larger bit line current is required, but the line has a lowerresistance and superior signal-to-noise ratio. The SAL is formed of soft(low coercivity) and highly permeable magnetic material such as alloysof Co, Ni and Fe and has a thickness between approximately 30 and 500angstroms which should be less that 5 times the thickness of the freelayer. A seed layer (30) is formed on the bit line and promotes the highquality crystalline formation of subsequently formed layers of the cell.The seed layer can be a layer of NiCr, NiFeCr or NiFe formed to athickness between approximately 20 and 100 angstroms. A single pinnedlayer or, as in this embodiment, a synthetic ferrimagnetic pinned layeris formed on the seed layer. The antiferromagnetic layer (32) pins themagnetization of the second ferromagnetic layer (33) unidirectionallyand the second ferromagnetic layer is magnetized in an antiparalleldirection to that of the first layer (35). The first and secondferromagnetic layers are layers of CoFe or CoFeB formed to thicknessesbetween approximately 10 and 100 angstroms and matched so that the netmagnetic moment of the configuration is substantially zero. The couplinglayer (34) is a layer of Rh, Ru, Cr or Cu of proper thickness tomaintain strong antiparallel coupling. The antiferromagnetic pinninglayer (32) can be a layer of PtMn, NiMn, OsMn, IrMn, NiO or CoNiO ofthickness between approximately 40 and 300 angstroms.

A tunneling barrier layer (36) is formed on the first ferromagneticlayer (35) of the pinned layer. This layer is a layer of insulatingmaterial such as oxidized Al or an oxidized Al—Hf bilayer and is formedto a thickness between approximately 7 to 15 angstroms. A ferromagneticfree layer (38) is formed on the barrier layer. At this stage of thecell fabrication, it is important to note that the vertical spacingbetween the SAL and the free layer should less than ⅕ the lateraldimension of the free layer. The free layer can be a single layer offerromagnetic material, such as a layer of CoFe, CoFeB or NiFe formed toa thickness between approximately 10 and 100 angstroms or it can be amultilayer, comprising first and second ferromagnetic layers, magnetizedin antiparallel directions and separated by a spacer layer of nonmagnetic but conducting material such as Rh, Ru, Cr or Cu, which is ofthe proper thickness to maintain strong antiparallel coupling betweenthe two ferromagnetic layers. A capping layer (40) is formed on the freelayer. The capping layer can be a layer of Ru, or Ta formed to athickness between approximately 10 and 1000 angstroms.

After the deposition of the capping layer, the MRAM cell is patterned toproduce a uniform horizontal cross-section which is triangular withrounded vertices or distorted circular with a large difference betweenupper and lower radii of curvature. As has been noted, this shapeasymmetry of the cell produces a corresponding magnetic anisotropy inthe free layer that promotes a two state magnetostatic interaction withthe SAL. It is also possible to provide an appropriate magneticanisotropy to the free layer during the annealing thatantiferromagnetically pins the pinned layer.

A layer of insulating material (100) surrounds the cell and separatesthe upper portion of the cell from the word line. It is noted that theword line is a layer of conducting material less than 100 nm inthickness and may be augmented with a cladding layer (44) of magneticmaterial formed on its surface away from the cell.

As has already been noted, the cell and cell array formed in accordancewith the preferred embodiment may be used to achieve the objects of theinvention (ie., locking, unlocking and state switching) by a particularsequence of currents in the word and bit lines. Referring to FIG. 3,there is shown a preferred sequence of word line and bit line currentsthat will achieve the objects of the present invention when used withthe MTJ MRAM cell of the present invention. Referring also to FIGS. 4a-d, there are shown states of the free layer corresponding to thecurrents in the word and bit lines of FIG. 3.

To understand the operation of the current sequence, we will assume thata certain cell has already been written upon and is presently storing,in a stable C state, a particular bit of information (a logical 1 or 0),corresponding to a particular magnetization state of the free layer. Thecell denoted (50) in FIG. 1 b, is an example of such a cell. The stateof the cell's free layer is also shown schematically in FIG. 4 a. Thecentral arrow (18) in the cell pointing along +y, represents thesubstantial magnetization direction of the cell produced by itsmagnetostatic coupling to the SAL. The smaller arrows at the top (19)and bottom (17) of the cell represent local magnetization at the upperand lower edges and stabilize the cell against unwanted switching of thecentral arrow. The present C state of the cell, therefore, hasguaranteed that it could not have been improperly switched (ie.,half-selected) by magnetic fields not directly addressing the cell priorto the present writing process.

The cell is now selected so that it can be rewritten, ie, that itspresent state of magnetization can be changed. The sequence of currentsrequired for this process must first place the cell into a less stable Sstate, so that its magnetization direction can be switched, then itsmagnetization is switched, then, finally, it is placed, once again, intothe more stable C state so that it remains thermally stable until it isselected again at some later time.

During a first portion of the rewrite process, shown in FIG. 3, there isa current in the word line, beginning at a time t₁, and in a firstdirection (called a positive current). For ease of visualization, wewill call this first direction, which is the positive current, the −ydirection (see FIG. 1 b for the appropriate directions). There is as yetno current in the bit line. The current in the word line induces amagnetic field in the SAL that is in the +x direction. This is true forevery cell that is below the active word line. The induced SALmagnetization couples with the magnetization of the free layer, and issufficient to rotate the upper magnetization (19) horizontally(clockwise) towards the +x direction, changing its state from stable C,to less stable S. This configuration is shown in FIG. 4 b. Note that theshape anisotropy of the cell makes only the magnetization of its narrowupper portion easy to shift clockwise, towards +x. The cell is nowunlocked and ready to be written on.

To rewrite the now unlocked cell, the bit line is activated at time t₂of FIG. 3 with a current in the +x direction and there is thensimultaneously current in both the word and bit lines at the location ofthe selected cell. Because it is in the S state, the small orthogonalmagnetic field produced by the bit line current is sufficient to switchthe free layer magnetization (18) of the selected cell to the −ydirection. The switched state is shown in FIG. 4 c. Note that when itwas originally in the C state (FIG. 4 a), the magnetic stability of thefree layer prevents it from being switched with the currents typicallycarried by word and bit lines.

After switching, the word line current is reduced so that beginning att₃ its current is constant and reversed (but of small magnitude) indirection to the +y direction. This current reversal is sufficient toswitch the direction of the upper magnetization of the free layer, butnot the lower magnetization, so that the free layer is placed in a Cstate (FIG. 4 d). Because of the shape induced magnetic anisotropy ofthe free layer, the small reversed current in the word line only changesthe magnetization of the free layer at its upper portion, which causesthe switch from S to C state. Finally, the bit line current is turnedoff at t₄ and the word line current is turned off at t₅, leaving thefree layer with its magnetization now in the −y direction and in astable C state which is maintained by the magnetostatic interaction withthe SAL. Note that the switching from C to S and from S to C occurs whenonly the word line is activated. The switching of the magnetizationdirection occurs only when both word and bit lines are activated.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than being limiting of the present invention. Revisionsand modifications may be made to methods, processes, materials,structures, and dimensions through which is formed an MTJ MRAM cell oran MRAM array of such cells, having a composite bit line with anadjacent soft magnetic layer that couples magnetostatically to the cellfree layer to make it thermally stable and capable of being locked orunlocked in accordance with whether or not it has been selected to bewritten upon, while still providing such an MTJ MRAM cell or array ofsuch cells, formed in accord with the present invention as defined bythe appended claims.

1. A method of selecting and writing on an MTJ MRAM cell having freelayer magnetization states of greater and lesser stability in each oftwo opposite directions and said cell being in an initial magnetizedstate of greater stability in one of said two directions, comprising:providing a substantially constant current, I₁, having a first directionand a first magnitude in a word line formed above said cell, saidcurrent beginning at a time t₁, and said current placing said cell in astate of lesser stability while leaving said magnetization directionunchanged; providing a substantially constant current having a seconddirection, which is orthogonal to said first direction and a secondmagnitude, I₂, in a bit line formed orthogonally to said word line belowsaid cell, said current beginning at a time t₂ wherein t₂ is greaterthan t₁ and said current reversing the direction of the magnetization ofsaid cell; providing, in said word line at a time t₃, which is greaterthan t₂ a substantially constant current having a third direction, whichis opposite to said first direction and a third magnitude, I₃, in saidword line, said current placing the free layer of said cell in a stateof greater stability, while leaving its magnetization directionunchanged; reducing the current in said bit line to zero at a time t₄which is greater than t₃; reducing the current in said word line to zeroat a time t₅, which is greater than t₄, whereby said free layer is in astate of greater stability with a magnetization direction that isopposite to that of said initial magnetized state.
 2. The method ofclaim 1 wherein I₁ is between approximately 1 and 10 mA, I₂ is betweenapproximately 0.5 and 5 mA and I₃ is between approximately 0.2 and 2 mA.